The present invention relates to a capacitance type acceleration sensor. Such capacitance type acceleration sensors can be mounted on a vehicle and serve to control vehicle systems in response to sensed acceleration conditions, such as a vehicle air-bag deployment system and a vehicle driving and braking system.
Various types of acceleration sensors have been proposed in the past, including pressure type sensors and strain gage type sensors. Capacitance type acceleration sensors of the type contemplated by the present invention exhibit excellent accuracy characteristics over wide temperature ranges.
Commonly owned U.S. Pat. No. 5,095,752 discloses a capacitance type accelerometer of the type the invention is directed toward improving. The contents of this U.S. Pat. No. 5,095,752 are incorporated herein by reference thereto for the purpose of showing the background of the present invention and the basic operating principals of capacitance type acceleration sensors. Published Japanese patent application 1-253657 also relates to a prior art capacitance type acceleration sensor. Reference is also made to a publication titled Semiconductor Capacitance - Type Accelerometer With PWM Electrostatic Servo Technique, presented at the SAE International Congress and Exposition, Detroit, Mich., Feb. 25-Mar. 1, 1991, printed as SAE Technical Paper Series 910274, for a discussion of capacitance type acceleration sensors which the present invention is directed toward improving.
FIG. 1 schematically depicts a circuit diagram for a conventional capacitance type acceleration sensor of the type referred to above, and FIG. 2 schematically depicts a prior art detecting unit or gage unit for the FIG. 1 sensor system. Referring to FIG. 1, acceleration G is detected with a detecting unit ("gage unit") 1, the signal being fed to an electronic circuit 2 for detecting electrostatic capacitance changes (.DELTA.C), the output of which circuit 2 is further processed with a holding circuit 3 and an adjusting circuit 4 to obtain an output voltage V.sub.o at terminal 13 which is directly proportional to the acceleration G.
The gage unit 1 has a movable electrode 5 serving as a weight interposed between upper and lower static electrodes 6 and 7, the movable electrode 5 being supported at a bendable beam between the static electrodes 6 and 7.
Since the static electrodes 6 and 7 and the movable electrode 5 are facing each other in substantially planar relationship, there exists electrostatic capacitances C.sub.1 and C.sub.2 therebetween, the values of these capacitances C.sub.1 and C.sub.2 being fed to one of the terminals of an operation amplifier 10 of the .DELTA.C detector unit 2.
When an acceleration G is applied to the gage unit 1, the movable electrode 5 is moved by inertia due to the acceleration (upward or downward as seen in the FIG. 1 illustration). Therefore, the distance between the movable electrode 5 and each of the electrodes 6 and 7 has changed with consequent changes in the electrostatic capacitances C.sub.1, C.sub.2. The .DELTA.C detector unit 2 operates so as to detect the differences C.sub.1 -C.sub.2 (.DELTA.C) using both generators 8, 9, a capacitor 11 for charge integration and a switch 12 for discharging. A voltage directly proportional to .DELTA.C is obtained from the amplifier 10 as an output. Since the voltage is not always kept constant over time due to the effect of the detecting operation described above, the holding circuit 3 is provided to modulate the voltage and provide an analog voltage V.sub.o directly proportional to the acceleration G. Since the present invention is not directly related to the details of the operation of this circuit, further details of the operation of this .DELTA.C detector unit 2 are not included herein, reference being made to the above noted prior art publications, and other prior art disclosures readily available to those skilled in the art.
FIG. 2 shows a prior art structure of a conventional gage unit 1 for use with the system of FIG. 1, the FIG. 2 gage unit 1 being similar to the FIG. 25 embodiment of the above-mentioned U.S. Pat. No. 5,095,572. Referring to FIG. 2, a movable electrode comprises a weight 5 which serves as the movable electrode for detecting capacitance. The weight 5 is supported by way of an integrally formed beam 14 and weight support 20 (weight support portion 21 is also part of the weight support connected to the weight support 20 in front and back of the plane of the FIG. 2 illustration and not shown in this Figure). Weight support 20, 21 is fixed to support members of glass plates 22 and 23 disposed at the top and bottom thereof as shown in the illustration of FIG. 2.
A static electrode 6 is placed on the side of the glass plate 22 facing the movable electrode 5 and is connected to an external electrode 16 through a through hole 15 formed by boring a hole through the glass plate 22. The structure of the glass plate 23 at the lower side is similar with a static electrode 7 placed on the side of the glass plate 23 facing the movable electrode 5 which static electrode 7 is connected to an external electrode 19 by way of a through hole 17 formed by boring a hole in the glass plate 23.
A problem with the prior art capacitance type acceleration sensor described above is that technical difficulties are encountered in precisely forming the through holes in the glass plates for extending lead wires out from the static electrodes and in attaching the lead wires. These difficulties have created significant problems in the production of sensors of this type.
An object of the present invention is to provide a capacitance type acceleration sensor of the general type described above, but where the connection of the lead wires to the static electrodes is greatly simplified. Another object of the present invention is to provide a capacitance type acceleration sensor of the above-noted type, wherein the detector circuit and the movable electrode are incorporated together as a single unit. A further object of the present invention is to provide new and improved methods of manufacturing a capacitance type acceleration sensor. These and other objects are achieved according to the present invention by providing a capacitance type acceleration sensor comprising a movable electrode which is movable in response to acceleration, a first static electrode facing the movable electrode, and a first solid dielectric member disposed between the movable electric and the first static electrode. In this sensor arrangement, the solid dielectric member serves to position and support the static electrode, without requiring that a hole be drilled through the solid dielectric member as is the case with prior art arrangements discussed above.
In especially preferred embodiments, the movable electrode is formed on a silicon plate member which is supported between plates or sheets which form first and second solid dielectric members that also support respective static electrodes on their respective sides facing away from the movable electrode. In this manner, the silicon plate member serving as the movable electrode is reliably supported in position between the solid dielectric members and the static electrodes are reliably connected to the solid dielectric members.
In especially preferred embodiments, the above-mentioned object of achieving a simpler construction with the unitary movable plate member and detector circuit, the capacitance type acceleration sensor comprises a unitary monocrystalline plate member with a movable cantilever plate portion forming a movable electrode which is movable in response to acceleration, a first static electrode facing the movable electrode, and an integrated circuit in said unitary plate member for forming an analog signal reflecting acceleration forces based on changes in capacitance between the movable cantilever plate portion and the first static electrode.
The preferred methods of making the capacitance of the acceleration sensor utilize the unique configuration with the static electrodes mounted on sides of the dielectric support members facing away from the movable electrode. Also, with unitary monocrystalline plate members forming the movable electrode and containing the integrated circuit for processing changes in capacitance caused by acceleration induced movement of the movable electrode to generate output signals correspondingly to acceleration of a vehicle carrying the sensor.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.